Manual therapies such as joint mobilization/manipulation have been shown to affect motoneuron pool excitability. The manual intervention can be directed at the local site of joint dysfunction or distant sites such as the lumbopelvic region. Joint mobilization/manipulations of the knee have been utilized as a treatment intervention for a variety of knee pathologies. Similar effects at the knee have been noted by treating the joints of a lumbopelvic region. A sacroiliac (SI) joint manipulation has been utilized with individuals with anterior knee pain and has been shown to acutely reduce pain when performing squats, step ups, and step downs, increase quadriceps muscle activation, and improve force output. The spinal mechanisms responsible for the changes in muscle activation following manual therapies are unknown. If joint manipulation can affect the afferent or efferent information mediated by the interneuron then it is possible that motoneuron pool excitability may be modulated (facilitated or inhibited). Motoneuron pool excitability can be modulated by both pre-synaptic and post-synaptic spinal mechanisms. The overall aim of this project is to determine the effect of local and distant mobilization/manipulation interventions on pre-synaptic and post-synaptic inhibition of the quadriceps muscle in individuals with existing quadriceps inhibition. This will be a double-blinded randomized controlled trial in which 75 subjects will be randomly assigned to receive one of four manual therapy interventions or no treatment and compared on measures of quadriceps muscle activation. Fifteen subjects will be assigned to receive a sacroiliac manipulation (Grade IV), 15 will receive sacroiliac positioning (Grade II), 15 will receive a patellar mobilization (Grade IV), 15 will receive patellar mobilization (Grade I), and 15 will receive no treatment (control). Subjects will be randomly assigned to treatment groups using concealed allocation. Outcomes will be the measurements of peak-to-peak H-reflex, paired reflex depression, and recurrent inhibition over time. We hypothesize that the sacroiliac manipulation (Grade IV) and patellar mobilization (Grade IV) interventions will result in a larger disinhibitory/facilitory representations of H-reflex, paired reflex depression and recurrent inhibition than the sacroiliac positioning only (Grade II), patellar mobilization (Grade I) and control conditions. We predict that the sacroiliac manipulation will result in greater post-synaptic disinhibition than the patellar mobilization, but the patellar mobilization will result in greater pre-synaptic disinhibition than the sacroiliac manipulation. We further hypothesize that these disinhibitory changes will continue to be present 30, 60, and 90 minutes after the treatments. Understanding how joint manipulation affects pre- and post-synaptic muscle inhibition will lay the foundation for our understanding of the mechanisms of manual therapies on motoneuron pool excitability and guide future development of optimal treatment interventions for joint pathologies. PUBLIC HEALTH RELEVANCE: This study will help elucidate the mechanisms responsible for improved muscle function following manual therapy interventions in people with muscle weakness. The specific effects from performing the treatment near the affected body part or at the lower back/pelvic region will be determined. This information will help develop optimal use of manual therapy treatments in people with muscle weakness secondary to injury.